1. Field of the Invention
The present invention relates to television tuners.
2. Description of the Related Art
As shown in FIG. 3, a tuner includes a UHF circuit 53, a VHF circuit 54, an integrated circuit 55, a phase locked loop (PLL) 56, a filter 57, a UHF resonant circuit 58, and a VHF resonant circuit 59. In the UHF circuit 53, a UHF antenna circuit 60, a UHF high-frequency amplifying circuit 61, and a UHF tuning circuit 62 are connected in series in this order from an input terminal 52 connected to an antenna 51. In the VHF circuit 54, a VHF antenna circuit 63, a VHF high-frequency amplifying circuit 64, and a VHF tuning circuit 65 are connected in series in this order from the input terminal 52.
The integrated circuit 55 includes a UHF mixer 66, a VHF mixer 67, a UHF local oscillator 68, a VHF local oscillator 69, a local-oscillator switching unit 70, a band-switching unit 71, and an intermediate-frequency amplifier 72.
A first input terminal of the UHF mixer 66 is connected to the output terminal of the UHF circuit 53, and a second input terminal of the UHF mixer 66 is connected to the UHF local oscillator 68. Intermediate-frequency signals from the UHF mixer 66 are output to the band-switching unit 71. A first input terminal of the VHF mixer 67 is connected to the output terminal of the VHF circuit 54, and a second input terminal of the VHF mixer 67 is connected to the VHF local oscillator 69. Intermediate-frequency signals from the VHF mixer 67 are output to the band-switching unit 71.
The intermediate-frequency signals pass through the band-switching unit 71 to be input to the filter 57. The filter 57 is a bandpass filter having steep characteristics, and the output terminal of the filter 57 is connected to the input terminal of the intermediate-frequency amplifier 72 in the integrated circuit 55. The output terminal of the intermediate-frequency amplifier 72 serves as an output terminal 73 of the tuner.
The PLL 56 applies a UHF-band switching voltage Vu for selecting a UHF band and a tuning voltage Vt corresponding to a selected channel to the UHF circuit 53. The PLL 56 applies a VHF-high-band switching voltage Vhi for selecting a VHF high band and a VHF-low-band switching voltage Vlo for selecting a VHF low band to the VHF circuit 54 and the VHF resonant circuit 59. The PLL 56 also applies the tuning voltage Vt to the VHF circuit 54. Moreover, the PLL 56 applies a control voltage to the local-oscillator switching unit 70 and the band-switching unit 71 in the integrated circuit 55. Oscillation signals from the UHF local oscillator 68 or the VHF local oscillator 69 are input to the PLL 56 through the local-oscillator switching unit 70, and the PLL 56 applies a control voltage to the UHF resonant circuit 58 and the VHF resonant circuit 59, depending on the frequency of the oscillation signals, and adjusts the oscillating frequencies of the UHF local oscillator 68 and the VHF local oscillator 69 (for example, see Japanese Unexamined Patent Application Publication No. 2000-295539, especially FIG. 3).
Since the input terminals of the two mixers are disposed close to each other in the integrated circuit, stray capacitance between the input terminals cannot be ignored. Thus, when a VHF band is received, harmonics (especially, those in the UHF band) of oscillation signals from the VHF local oscillator leak into the input terminal of the UHF mixer through the input terminal of the VHF mixer, and are further transmitted through an inactive UHF circuit in the reverse direction to be unnecessarily radiated from the antenna.